1. Field of the Art:
The present invention relates generally to a ring-or loop-type or a unidimensional or linear signal transmission system and, more particularly, to a transmission line control system capable of disconnecting a terminal communication apparatus from or reconnecting it to a main transmission line reliably and speedily in such occasions as fault, release, turning on or off of the power source.
2. Prior Art:
Ring- or Loop-Type Transmission Line Control
FIG. 1 is a schematic circuit diagram illustrative of a ring- or loop-type transmission line control, in which terminal transmission apparatuses S.sub.1, S.sub.2, S.sub.3 and S.sub.4 are connected in a ring-shaped configuration by transmission lines L. By-passes BP.sub.1, BP.sub.2, BP.sub.3 and BP.sub.4 are added to the transmission lines L in parallel to the communication apparatuses S.sub.1, S.sub.2, S.sub.3 and S.sub.4.
Any one of the communication apparatuses can transmit signal to and receive a signal from any of the other communication apparatuses through the ring arrangement of transmission lines L.
The difference between the ring-type and the loop-type will now be described.
The term "ring-type" as used herein is to be understood to mean a transmission control system in which all the communication apparatuses are on the same state with respect to communication control and are controlled independently without any particular apparatus which acts as the center in control.
The term "loop-type" as used herein is to be understood to mean a transmission control system in which a specified communication apparatus has the function of communication control to perform all the control operations as the main apparatus.
While there is a difference in the main control apparatus between the ring-type and the loop-type as mentioned above, they are equivalent to each other in the configuration of the transmission lines.
Since the communication apparatuses are connected in series, when a fault occurs in any of the comminication apparatuses or in a line leading thereto, it is necessary to disconnect the faulty communication apparatus and to close the by-pass switch, to thereby prevent the ring transmission lines from being interrupted.
In the arrangement of FIG. 1, if a line fault R occurs in the communication apparatus S.sub.1, a by-pass BP.sub.1 is closed. Then, the communication apparatuses S.sub.4 and S.sub.2 are connected with each other and the apparatus S.sub.1 is disconnected from the ring (loop). If the power source of the communication apparatus S.sub.3 is cut off (denoted by Q), a by-pass BP.sub.3 is closed and the apparatus S.sub.3 is also disconnected from the ring (loop).
In case of occurrence of such fault or disconnection of the power source, the transmission line must be switched in order to maintain the loop transmission and the switching was heretofore performed either by
(a) switching the transmission lines manually, or by
(b) applying a control signal separately from the transmission data to the transmission lines for control.
The method (a) in which the transmission lines were switched manually had disadvnatages such as low reliability and slow response.
The method (b) will be described below.
Control Data Overlapping System
FIG. 2 is a block diagram illustrating the conventional transmission line control system. While FIG. 2 shows only one communication apparatus branched from the transmission line L, it must be understood that all the communication apparatuses are of the same configuration.
A transmission line side circuit U is provided on the side of the transmission line L, and subsidiary transmission lines M are provided to connect between the transmission line side circuit U and a terminal side circuit T.
The terminal side circuit T equivalent to the communication apparatus of FIG. 1 generates a data signal DS and a control signal CS, and overlaps and applies these signals as one signal to the subsidiary transmission line M. The transmission line side circuit U receives the overlapped signal and separates it into the data signal DS and the control signal CS. The transmission line and the terminal side are coupled to or separated from each other by the control signal CS.
Transmission of the signal from the transmission line side circuit U to the terminal side circuit T is performed in the same manner. That is, the data signal DS and the control signal CS are overlapped in the transmission line side circuit U and applied through the subsidiary transmission line M to the terminal side circuit T where they are separated back to the data signal DS and the control signal CS.
The data signal DS containing a large number of data comprises rapidly repeated pulses. The control signal CS changing in value only in case of some fault, failure in power source or other trouble is repeated very slowly and irregularly.
The conventional transmission line control system described above had the following disadvantages:
(a) Circuit was complicated--because an additional circuit was necessary for generating, overlapping and separating the signals; and
(b) Reliability was low--because an additional control signal besides the data signal to be transmitted had to be sent through the transmission lines.
Accordingly, an object of the present invention is to provide a transmission line control system capable of switching the transmission lines reliably, quickly and automatically using only a data signal without using a control signal.
Another object of the present invention is to provide a transmission line control system in which, in the case where the power source is disconnected in a terminal apparatus, transmission is performed in the steady state among the remaining terminal apparatuses with said one apparatus excluded.
Still another object of the present invention is to provide a transmission line control system in which, in the case where the once disconnected power source of said terminal apparatus is connected again, said terminal apparatus is restored to the steady state when it is in order, or said terminal apparatus is not connected back to the main transmission lines when it is out of order.
Yet another object of the present invention is to provide a transmission line control system in which, in the case where a subsidiary transmission line fails, the terminal apparatus related to the fault is disconnected from the main transmission line so as to maintain the other terminal apparatuses in normal operation.
A further object of the present invention is to provide a transmission line control system in which a repaired terminal apparatus is reconnected to the steady state by manually closing an initially set switch.